Taiwan is an Island in the western Pacific Ocean and experienced more than 3 typhoons in a year. During Typhoon periods, it always brings intense rainfall, high waves, and storm surges, which often resulted in coastal flooding. The flooding can be aggravated by the sea level rise due to the global warming, which may subject Taiwan's coastal areas to more serious damage in the future than the present. The objectives of this study are to investigate the flooding caused by typhoons in Tamsui River, a river located in the north of Taiwan by numerical simulations, considering the effects of sea-level rises according to the level suggested by the 5th Assessment Report of IPCC (Intergovernmental Panel on Climate Change) for 2050 and 2100, respectively. The simulations were carried out by using MIKE21 HD (a hydrodynamic model) and MIKE21 SW (a spectral wave model). Our preliminary results showed that with only the effect of rainwater discharge, the maximum surface level of the river during the storm near the estuary is lower than the river level during the storm surge condition. With the storm surge and sea-level rise, the maximum river levels near the estuary were 3.4 ~ 3.9 m. When the flow rate increases upstream of the Guandu Bridge, the water level can be found near the mouth of the Tamsui significantly raising the average water level is about 3.7 ~ 4.3m. This showed that due to higher sea-level, the area near the estuary will be flooded.
Sea level rise has strengthened the hydrodynamic effects of nearshore, further aggravated the problems of coastal erosion or the original ones, and especially the sandy coasts. Coastal erosion causes the loss of coastal sandy beach space, the shrinkage and reduction of coastal buffer zones, the damage of coastal structures and even the loss of land and urban space. In addition, rising sea levels have also caused salt water intrusion, deteriorating water quality, rising groundwater levels, and even the destruction of ecological environment and resources. Sea level rise directly affects land runoff and groundwater quality in the coastal plains. Sea water intrudes inland via rivers, making salty water quality at the estuary, affecting urban water supply and industrial and agricultural water use, while causing interference with existing drainage and irrigation systems Irrigation and drainage poor and cannot be used (Chen, 2001).
